Cystic fibrosis (CF) is the most common inherited life-shortening disease among white populations, and is characterized by progressive loss of pulmonary function, exercise tolerance, and quality of life. Previous short-term studies have shown that patients with CF may benefit from aerobic exercise, with improved cardiopulmonary fitness, exercise tolerance, and perhaps either improvement or delay in the expected deterioration in lung function. Recent observations have confirmed a strong correlation between aerobic fitness and survival in patients with CF There have been no published studies that used behavioral medicine techniques to secure compliance with a long-term aerobic exercise program for patients with CF, aged 8 - 18 years, to see if the benefits, particularly the increase in aerobic fitness, seen with short programs could be sustained. This proposal would institute a controlled randomized study of one year of at-home aerobic exercise (stair-stepping) compared with at-home upper body weight-lifting exercise (that is unlikely to increase aerobic fitness) for patients with CF. The study design incorporates state-of-the-art behavioral medicine techniques and a strong money-based incentive system to secure patients' continued participation. The effects of such a program on aerobic fitness (measured as peak oxygen consumption), exercise tolerance, pulmonary function, upper body strength, and quality of life will be measured. This intervention has the potential to increase CF patients' fitness, pulmonary function, life-quality, and even longevity.